24 T. H. BüRLEND, 



The segments affected by the kidney development at this stage 

 are: V?, VI?, VII, VIII, IX, two-thirds of X; the duct extends 

 over the post, one-third of X, XI, XII, XIII and 10 sections 

 further back. 



On the right side the groove begins at 3,17, and is continued 

 back into a solid outgrowth which extends to 4,7. 



The duct reaches from 4,7 as far back as 5.13. 



Hence the kidney region on the right side includes : post, region 

 of V, VI, VII, VIII, IX (ant. half), and the duct extends over IX 

 (post, half) X, XI, XII, XIII and 6 sections further back. 



Comparison 



of Embryos A, B, C, D, E, F, G and H with regard to 



the development of the kidney and its duct. 



At this stage it is well to summarise the results which an 

 examination of the above embryos yields. In the first place, the 

 condition of the kidney at its most anterior end is in the form of a simple 

 evagination of the somatic layer of the lateral mesoderm so as to pro- 

 duce a groove which is continuous and unsegmented. This groove 

 I propose to call the "primitive kidney groove". The furthest 

 anterior extent of this groove is: 



on left side in segment III or II? 



on right side in segment II. 



Moreover, the kidney or its duct seems to be always a little 

 further advanced in development posteriorly on the left side as 

 compared with the right side. 



The kidney rudiment which we have up to the present described 

 becomes the future pronephros and its duct. Thus the duct and 

 the rudiment of the pronephric tubules arise simultaneously in the 

 first few segments where the kidney occurs in these embryos. The 

 further development is accompanied by atrophy of the kidney anteri- 

 orly in any or all of the first 5 or 6 segments. Posterior development 

 comprises the formation of a duct — in continuation with the 

 rudiment already formed — and (as all evidence tends to prove) not 

 by increase of its own cells only — viz. independent backward 

 growth, but by growth due to proliferating cells from the somatic 

 layer of the mesoderm. 



After the duct is formed, a general fusion with the somatic layer 



